1. Field of the Invention
The present invention relates to radar equipment for measuring distance or speed, and more particularly radar equipment exemplarily mounted on a running vehicle, measuring a relative distance or a relative speed between the vehicle and another vehicle running ahead or behind the vehicle, by use of the spread spectrum communication scheme.
2. Description of the Related Arts
When a vehicle (an automobile in particular) is running, in order to avoid collision against a preceding vehicle running ahead of the vehicle of interest, it is necessary to keep a certain distance to the preceding vehicle to run safely. Recently, a vehicle equipped with a distance measurement apparatus to a preceding vehicle has come into practical use. When the measured distance to the preceding vehicle becomes shorter than a predetermined distance, the apparatus issues an alarm to the driver. Furthermore, automatic drive control for keeping a certain distance to the preceding vehicle also becomes available.
A known apparatus for measuring a distance or a relative speed to the preceding vehicle is radar equipment, in which a spread-spectrum communication scheme is applied. The spread spectrum communication transmits a continuous spreading code such as an M-sequence pseudo noise (PN) code. The distance and the relative speed to a target object are measured by performing correlation detection processing on a reception signal reflected at the target. Since the spread spectrum communication has advantages of enabling continuous measurement without producing interference with other apparatuses, the spread spectrum communication has actively been studied and developed.
For example, in the official gazette of the Japanese Unexamined Patent Publication No. 2000-9838, collision-preventive radar equipment for automobile using the spread spectrum communication scheme is disclosed. According to the above disclosure, the equipment is provided with a plurality of correlation detection processing systems so as to capture and track a plurality of targets simultaneously and independently. Further, an analog-to-digital converter (ADC) is provided for digital correlation processing.
However, when handling a high-speed signal of which spreading code speed exceeds, for example, 1 Gbps, a high sampling rate is required for the ADC. This produces a problem of a larger ADC circuit in scale, as well as an increased cost and consumption power.
Also, in the official gazette of the Japanese Unexamined Patent Publication No. Hei-5-256936, on-board radar equipment using the spread-spectrum communication scheme is disclosed. According to this disclosure, the equipment includes multi-stage delay units. A demodulated signal is input into an exclusive-OR circuit. Digital correlation processing is performed in parallel without using an analog-to-digital converter, by use of a plurality of fixed delay amounts which are mutually different.
However, in order to increase accuracy and resolution in the measured distance and relative speed, a delay amount step must be set smaller, which causes an increased circuit scale. Further, a demodulated signal level varies depending on the distance to the target, and when the demodulated signal level becomes out of a tolerable input level range of the exclusive-OR circuit, the correlation processing cannot be performed and the distance cannot be measured.